In recent years, a full color recording system for directly recording an image photographed with a video camera, an image on a TV, a video tape recorder, a video disk, a computer, etc. on a recording material has been extensively developed. In particular, attention has been aroused on a recording system in which a support coated with a coloring material which is melted, evaporated, or sublimated by application of heat is superposed on a recording sheet (image-receiving sheet), and the coloring material is heated with a thermal head according to recording signals and then transferred to the recording sheet to form an image through adhesion, adsorption, or dye-fixing. One of the admitted characteristics of this recording system is that plain paper or a synthetic resin film, etc. may be used as the image-receiving sheet.
However, when plain paper or a resin film is used as the image-receiving sheet, the dye-fixing, in particular, is difficult to accomplish. As a result, not only is the resulting recorded image low in color density, but serious fading occurs with time.
It has been therefore suggested to coat a support of an image-receiving sheet with a thermoplastic resin, e.g., a polyester resin, to form an image-receiving layer. However, since a thermal recording head of a thermal dye-transfer recording device is generally heated to 200.degree. C. or higher, an ink binder in the coloring material-transferring sheet and the thermoplastic resin in the image-receiving layer are softened or melted by heat to cause fusion therebetween. As a result, the both sheets are difficult to peel apart after recording or the ink layer itself of the coloring material-transferring sheet is transferred to the image-receiving sheet (this phenomenon is hereinafter referred to as blocking).
In order to prevent fusion between the coloring material-transferring sheet and the image-receiving sheet, it has been proposed to incorporate a pigment into the resin constituting the image-receiving layer to rough the surface of the image-receiving layer as disclosed in JP-A-57-107885 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application"); to coat a release agent, e.g., silicone grease, on the image-receiving layer as disclosed in JP-A-59-165688; or to incorporate a release agent, e.g., a silicon compound, into the image-receiving layer as disclosed in JP-A-60-34898, JP-A-60-212394, and JP-A-61-237694.
In the former method using a pigment, however, contact between the coloring material-transferring sheet and the image-receiving sheet at the time of dye transfer is insufficient due to the surface roughness, resulting in reduction of image density or color unevenness. In addition, the dye adhered onto the surface of the pigment tends to stain other materials, such as paper. The latter method using a release agent is also disadvantageous in that the transferred dye is affected by the release agent, causing smearing of the recorded image or reduction of image density.
Other approaches include a method in which a mixture of a thermoplastic resin and a radical polymerizable compound is cured to form an image-receiving layer having improved heat resistance as disclosed in JP-A-58-212994 and a method in which a crosslinked heat-resistant release layer is provided on an image-receiving layer as disclosed in JP-A-62-116189. These methods have turned out, however, only to provide insufficient recording density if the curing or crosslinking is conducted to such a degree enough to prevent blocking.
Moreover, all the above-described methods use a coating composition containing a large amount of an organic solvent or water so that a drying means and a large quantity of energy are required for drying the solvent. In addition, the organic solvent gives rise to environmental, handling, safety and economical problems because of its harmfulness to human bodies, a fear of explosion, and expensiveness.
On the other hand, JP-A-62-173295 recommends a method using a solvent-free coating composition, in which a radical polymerizable oligomer whose cured product has a glass transition temperature of not higher than 65.degree. C. is coated on a support and crosslinked by irradiation. However, an attempt of obtaining high recording density by this method has turned out to give a recorded image which is liable to bleeding or lacks storage stability.